JP2563287B2 - Fuel assembly for nuclear reactor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Object of the Invention (Field of Industrial Application) The present invention relates to a fuel assembly for a nuclear reactor, and more particularly to a fuel assembly for a nuclear reactor suitable for high burnup.

(Prior Art) Fuel assemblies used in current commercial nuclear reactors are
Fuel elements filled with zirconia enriched uranium dioxide pellets are packed in a zirconium alloy cladding tube and arranged in a square lattice, and the fuel rods are bundled by spacers while keeping the distance between the fuel rods. There is. In addition, in order to control the excess reactivity of the fuel in some of the fuel rods that compose this fuel assembly, mixed oxide pellets of enriched uranium oxide and gadolinium oxide (gadolinia) that is a neutron absorbing substance are used. Is filled (hereinafter referred to as gadolinia rod).

FIG. 7 shows a layout of a conventional uranium fuel assembly for a boiling water reactor. In this fuel, fuel rods (represented by numbers in the order of increasing enrichment) are arranged in an 8 × 8 square lattice, and a water rod W through which cooling water flows is permanently arranged in the central portion. . And, of the 62 fuel rods, 8 are gadolinia rods, and the remaining 54 are ordinary fuel rods 1 to 4 filled with oxide pellets of enriched uranium.
FIG. 8 shows the combustion change of the infinite multiplication factor of the fuel of FIG. 7 described above. It increases with combustion from unburned to a burnup of 10 GWd / t, and decreases thereafter. When this fuel is loaded into a 3-batch replacement core with an operating period of 10 GWd / t (one-third of the total core is replaced with fresh fuel at the time of refueling), the infinite multiplication factor increases (first cycle fuel) The decreasing portion (fuel after 2 cycles) is just balanced, and a substantially flat effective magnification is achieved throughout the operation period.

Such fuel can be obtained by properly designing the number of gadolinia rods and the gadolinia concentration. The former decides how much the infinite multiplication factor is suppressed, and the latter decides to what extent. In particular, the gadolinia concentration must be determined so that the gadolinia is completely burned at the end of the operating cycle (hereinafter referred to as EOC). If gadolinia remains in the EOC, neutrons will be unnecessarily absorbed, and fuel economy will deteriorate.

(Problems to be Solved by the Invention) In recent years, from the viewpoint of improving fuel economy, a design in which the number of replacements per cycle is reduced by increasing the content of fissile material per fuel assembly has been advanced. . For this purpose, it is necessary to suppress the excess reactivity of the fuel assembly at the initial stage of combustion, so it is necessary to increase the number of gadolinia more than before.

On the other hand, since the gadolinia rod has a large neutron absorption capacity,
In order to avoid strong interference of neutron absorption between gadolinia rods, the conventional design idea is not to place them adjacent to each other in the fuel assembly, which limits the freedom of placement of gadolinia rods in the fuel assembly. Is coming.

However, if the number of gadolinia rods increases due to the high burnup as described above, there may be a case where the required number of gadolinia rods cannot be arranged because there is no place for arrangement in the assembly. As a countermeasure against this, if the gadolinia concentration is increased so that the gadolinia remains unburned at the EOC, the infinite multiplication factor at the beginning of the combustion of the second cycle fuel will be small, and
(Abbreviated as)), the reactivity of the core is reduced accordingly and the initial excess reactivity can be suppressed. However, on the other hand, there is a problem that the gadolinia is unnecessarily burned in the EOC, which reduces the reactivity of the core, shortens the operation cycle length, and lowers the fuel economy.

In addition, Fig. 6 is a layout diagram in which the location of gadolinia rods is particularly limited. As shown in the figure, approximately 1/3 of all fuel rods, 20 of which are plutonium-rich MOX ( Mixed Oxide) A so-called island type MOX fuel assembly is arranged in the center of the fuel assembly as a fuel rod. Although the one shown in the drawing is a design example corresponding to high burnup, a large diameter water rod W having a size corresponding to four fuel rods is arranged at the center of the fuel assembly.
Increasing the water to fuel volume ratio in the fuel assembly results in two fewer fuel rods than the conventional fuel shown in FIG. Here, the uranium fuel rods are represented by the numbers 3,2,1 in order of increasing enrichment, and the MOX fuel rods are represented by P. The gadolinia rod G uses uranium as a base material and does not contain plutonium in order to simplify the fuel manufacturing process.

When such a design is adopted, it is necessary to avoid the location of the gadolinia rod in the central portion. Further, in the conventional design method, the arrangement of the outermost periphery along the water gap of the gadolinia rod is avoided because the gadolinia combustion characteristics change more than those of the inner arrangement.

From the above, the place where the gadolinia rod is arranged is significantly limited. The MOX fuel assembly shown in the figure is
The reactivity life is equivalent to that of a uranium fuel assembly with an average enrichment of about 3.5 w / o, but more than 10 gadolinia rods were required to suppress the excess reactivity in the early stage of combustion. However, as shown in the figure, there is a problem that the gadolinia concentration is 3.0 w / o with a limit of 10 and the gadolinia rod G ₁ is designed to be left unburned by EOC. (Means for Solving the Problems) In order to solve the above problems, in the fuel assembly for a nuclear reactor of the present invention, at least two sets of gadolinia rods arranged in the fuel assembly are mutually in the X direction or the Y direction. Secures the required number by arranging in a position adjacent to
It is characterized in that the gadolinia concentration of at least one of the gadolinia rods adjacent to each other in the Y direction or the Y direction is reduced. Here, the positions adjacent to each other in the X direction or the Y direction mean positions adjacent to each other in the X direction or the Y direction of the square lattice array, that is, in the vertical or horizontal direction in the horizontal cross section of the fuel assembly. The diagonal adjacency of the array does not correspond to this. In addition, the L-shaped or T-shaped ones that are adjacent to each other in both the X and Y directions are counted as one set.

(Operation) Gadolinium ( ¹⁵⁵ Gd, ¹⁵⁷ Gd) suppresses the excess reactivity of the fuel assembly by absorbing the neutrons generated in the gadolinia rod and the neutrons flowing from the adjacent fuel rods. ^{Since 155} Gd and ¹⁵⁷ Gd absorb neutrons and change to ¹⁵⁶ Gd and ¹⁵⁸ Gd, which have a small neutron absorption cross section, the neutron absorption capacity of the gadolinia rod gradually decreases with burnup and is designed to burn out at EOC. There is.

When the number of gadolinia rods is increased and some of them are arranged adjacent to each other in the X direction or the Y direction, the excess reactivity at the beginning of the cycle can be suppressed. Absorption will be reduced and the gadolinia will burn late. As a remedy for this, if at least one of the gadolinia rods arranged adjacent to each other in the X direction or the Y direction is burned out in the first half of the operation cycle period, the gadolinia concentration will be sufficiently small so that the other gadolinia rods will not reach EOC. Since it burns out, fuel economy can be improved.

(Example) Hereinafter, the Example of this invention is described with reference to drawings.

FIG. 1 is a layout view of a MOX fuel assembly according to an embodiment of the present invention. As shown in the drawing, L-shaped gadolinia rods are arranged adjacent to each other at two locations, and the outermost circumference is provided to enhance neutron absorption. This is an example of arranging _two gadolinia rods G ₂ (concentration 1.5w / o) in the high thermal neutron flux part of the second to the corner part. By increasing the number of gadolinia rods, the initial excess reactivity was suppressed. , The gadolinia concentration of the gadolinia rod G ₁ could be reduced to 2.5 w / o.

FIG. 2 is a layout view of a MOX fuel assembly according to another embodiment of the present invention. As shown in FIG. 2, L-shaped gadolinia rods are adjacently arranged at four places, and eight gadolinia rods G ₁ are arranged. Reduced to
Instead, in the example in which G ₂ was increased to 4, the symmetry of the concentration / enrichment distribution in the fuel assembly was particularly improved, and the local output distribution was flattened as compared with the above-mentioned embodiment. Gadolinia concentration is G ₁
In 2.5w / o, G ₂ is a 1.5 w / o of 60% decline in the concentration of G _1, it was possible to further reduce the gadolinia amount of total.

FIG. 3 shows the MOX fuel assembly of the present invention shown in FIG.
FIG. 4 is a diagram showing a combustion change of an infinite multiplication factor of the conventional MOX fuel assembly shown in FIG. 4, and FIG. 4 shows surplus reactivity during power operation of the core of the present invention of FIG. 2 and the conventional core of FIG. This shows the room shutdown margin during cold temperatures. In this figure, the cold shutdown shutdown margin is set to an effective multiplication factor of 0.99 or less, while the design standard is satisfied, the excess reactivity during output operation at the initial stage of combustion is reduced to flatten the combustion change and the operating cycle length is extended. The situation is shown.

From this, according to the present embodiment,
It can be seen that the excess reactivity of the fuel can be suppressed and the gadolinia unburned residue at the EOC can be reduced to improve the fuel economy.

FIG. 5 is a layout view of still another embodiment of the present invention,
An example of a uranium fuel assembly with a higher burnup is shown. In this embodiment, a large diameter water rod W having a size equivalent to nine fuel rods is arranged in the center of the fuel assembly forming a fuel rod array of 9 rows and 9 columns, which is larger than that shown in the embodiment of FIG. Increasing water to fuel volume ratio. Here, the numbers 3,2,
It was placed a low enrichment fuel to 1 forward fuel assembly average enrichment has about 6w / o, the gadolinia rods G ₁ at a concentration 6.0 w / o
Twenty bars are arranged and eight gadolinia bars G ₂ with a concentration of 1.5 w / o are arranged.

In this embodiment, the gadolinia rods are L-shaped or T-shaped.
The low-concentration gadolinia rod G ₂ is arranged adjacent to the character shape at a position facing the corner of the second outermost row of the fuel assembly having a high thermal neutron flux and the water rod to enhance the thermal neutron absorption of the gadolinia.

In the embodiments described above, the gadolinia rods are adjacent to each other in the X direction or the Y direction, or in the L-shape or the T-shape, and the low-concentration gadolinia rods are used as the corners or water rods in the second row from the outermost periphery of the assembly. Although it is limited to the facing portion, it can be arranged at another place and the degree of freedom of the place can be increased by appropriately adjusting the number of gadolinia densities.

[Effects of the Invention] As described above, according to the reactor fuel assembly of the present invention, it is possible to arrange the required number of gadolinia rods in the fuel assembly and eliminate the unburned residue of gadolinia at the end of the operation cycle. It has an excellent effect that the fuel economy can be improved.

[Brief description of drawings]

FIG. 1 is a layout view of a MOX fuel assembly according to an embodiment of the present invention,
FIG. 2 is a layout view of the MOX fuel assembly of another embodiment of the present invention, and FIG. 3 is an infinite multiplication factor of the MOX fuel assembly of the present invention of FIG. 2 and the conventional MOX fuel assembly of FIG. FIG. 4 is a diagram comparing combustion changes, and FIG. 4 is a comparison of combustion changes of surplus reactivity during power operation and reactor shutdown margin of the MOX fuel assembly of the present invention of FIG. 2 and the conventional MOX fuel assembly of FIG. Figure,
FIG. 5 is a layout view of a uranium fuel assembly according to still another embodiment of the present invention, FIG. 6 is a layout view of a conventional MOX fuel assembly, and FIG. 7 is a conventional uranium fuel for a boiling water reactor. FIG. 9 is a layout view of the assembly, and FIG. 8 is a diagram showing a combustion change of the fuel of FIG. 7 at an infinite multiplication factor. 1-4 uranium fuel rod P …… MOX fuel rod G ₁ , G ₂ …… Gadolinia rod W …… Water rod

Claims (3)

(57) [Claims] 1. A fuel assembly in which a large number of fuel rods and water rods filled with pellet-shaped fuel elements are arranged in a square lattice, and combustible poisons are arranged at positions adjacent to each other in the X direction or the Y direction. A fuel assembly for a nuclear reactor, characterized in that there are at least two sets of incoming fuel rods, and at least one fuel rod in each set has a low concentration of burnable poisons. 2. A fuel rod position to which a low concentration of burnable poison is added is located at a corner portion of the second row from the outermost periphery of the fuel assembly or a position adjacent to the water rod. A fuel assembly for a nuclear reactor according to the item. 3. The fuel assembly for a nuclear reactor according to claim 1, wherein the concentration of the burnable poison at a low concentration is 60% or less of the concentration of the burnable poison at the maximum concentration. .